Скачать или смотреть Understanding Why the collect Method for Java Streams Doesn't Work as Expected

Discover why using `collect` with immutable `String` doesn't yield the expected results in Java streams and learn about the correct approach with practical examples.
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Why the collect Method for Streams Doesn’t Work as Expected

Java streams provide a powerful abstraction for processing sequences of elements. However, some developers may encounter unexpected behavior when using the collect method, especially with immutable objects like String. In this guide, we will explore a specific case where the collect method doesn't behave as expected and provide a comprehensive solution to this problem.

The Problem

Imagine you want to build a single String from an array of String elements using the collect method. The following Java code illustrates this:

[[See Video to Reveal this Text or Code Snippet]]

In this example, you would expect both System.out.println calls to produce the intended results by collecting strings into a single String. However, you notice that the second case, which uses String, outputs an empty string, leading to confusion.

Understanding the Issue

The key reason for this unexpected behavior is that String is immutable in Java. When you attempt to use String::new, which acts as a supplier that returns an empty String, it cannot be modified by the subsequent operations defined in your accumulator and combiner. Here’s what happens under the hood:

Immutable Nature of String: Unlike StringBuilder, which can change its internal state (mutate), String creates a new instance whenever you perform concatenation.

Behavior of concat: The concat method does not modify the existing String but instead returns a new String. Thus, the assignment occurs to a new String instance that never gets captured.

An analogy that illustrates this point is as follows:

[[See Video to Reveal this Text or Code Snippet]]

As a result, your final output remains the same as the initialized value — an empty string.

The Solution: Using reduce

To correctly concatenate strings where immutability of String is a factor, you can use the reduce method instead of collect. The reduce method is designed for comfortable handling of immutable objects. Here’s how you can modify your code:

[[See Video to Reveal this Text or Code Snippet]]

This change allows you to accumulate the concatenated result correctly, as it will iteratively build upon the existing string instead of creating new ones that are discarded. The equivalent iterative process would look like this:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

In summary, when encountering unexpected behavior with the collect method in Java streams, particularly with immutable types like String, remember that immutability prevents mutation of existing instances. Instead, use the reduce method for safe and effective string concatenation. By understanding this behavior, you can avoid similar pitfalls in your future Java projects.

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